During the late 1930s and early 1940's, Hans von Ohain of Germany and Frank Whittle of the United Kingdom introduced the first commercially viable gas turbine engines for aeronautical propulsion. Whittle and von Ohain worked independently in their respective countries without knowledge of the other's work. The resultant Whittle W.1 and the Heinkel HeS 3 were remarkably similar and both featured a centrifugal compressor.
The following sets out some terminology for gas turbine engines, and some classifications thereof. In particular, a gas turbine engine can, roughly-speaking, be described as having two sections: a cold section and a hot section. The hot section contains a combustor, turbine, and nozzle, whereas the cold section contains a propeller, which applies to turboprops, an inlet, and a compression system.
The compression system contains at least one compressor and, in turbofan engines, a fan. A particular compressor can be further designated as one of: low pressure, intermediate pressure, or high pressure, as determined by the spool to which the compressor is attached.
The designation for a spool depends on the turbine. For example, a spool attached to the high pressure turbine—the turbine closest to the combustor—is referred to as a high pressure spool. A spool attached to the low pressure turbine—the turbine furthest away from the combustor—is referred to as a low pressure spool. A spool attached to the intermediate pressure turbine—the turbine that is in between the high and low pressure turbines—is referred to as an intermediate pressure spool. Commercially speaking, a typical gas turbine engine incorporates two or three spools. If a gas turbine incorporates more than three spools, the same naming convention will still apply, meaning it will still depend on the turbine.
Logically, therefore, the compressor attached to a high pressure spool may be referred to as a high pressure compressor, the compressor attached to a low pressure spool may be referred to as a low pressure compressor, and the compressor attached to an intermediate pressure spool may be referred to as an intermediate pressure compressor.
A low pressure, intermediate pressure, or high pressure compressor may be further designated as having a particular design, depending on its respective components and number of stages. At present, there are three main types of compressors: those incorporating a radial impeller are known as centrifugal compressors, those incorporating a diagonal impeller are known as diagonal compressors, and those incorporating an axial rotor blade are known as axial compressors.
In the field of gas turbine engines, a compression system has many components. However, conventionally, a three-stage high pressure centrifugal compressor refers to a compressor having three radial impellers attached to a high pressure spool. As another example, a five-stage low pressure axial compressor refers to a compressor having five rows of axial compressor blades attached to a low pressure spool. As yet another example, a six-stage low pressure (5:1) axial-centrifugal compressor refers to a compressor having five rows of axial compressor blades and one radial impeller attached to a low pressure spool.
While the first aeronautical gas turbine engines used centrifugal compressors, they have been largely replaced by axial compressors. This is primarily because, while a single stage in a centrifugal compressor can compress airflow many times more compared to a single stage in an axial compressor, it is notoriously more difficult to produce an efficient multi-stage centrifugal compressor compared to a multi-stage axial compressor. In particular, efficient fluid transport from a centrifugal compressor or from another component that exits fluid centrifugally to a subsequent stage has heretofore been problematic as compared to fluid transport from components, such as axial compressors, that exit fluid axially.